Many applications require synchronization of distributed discrete components with a central timing reference. For example, a telecommunications switching system may include a large number of discrete components that are used to perform the switching operations and other telecommunications services. Synchronization of components is of particular importance in digital communications, where a difference in timing of less than a microsecond can disrupt effective data transfer.
A digital cross-connect (DCC) system is a specialized switching system that provides improved flexibility in switching services. An example of a modern DCC system is provided by U.S. Pat. No. 5,436,890 to Read et al., entitled "Integrated Multirate Cross-Connect System," assigned to DSC Communications Corporation, issued Jul. 25, 1995 (hereinafter "Read"). Such DCC systems may include a plurality of devices that define the M input ports and N output ports, an M.times.N connection matrix switch operable to connect any of the M input ports to any of the N output ports, and an administration subsystem that provides synchronization, monitoring, and control for remapping of the connection matrix. In addition, the DCC system taught in Read contains redundant parallel planes (i.e., circuit paths) of all components, such that the DCC system can experience a number of failures without adversely affecting network traffic.
The potentially large number of components in a DCC system with redundant parallel planes complicates the control of DCCs and the communication of data between components of the DCC system. Such systems utilize a timing signal of high accuracy in order to ensure that data communications are not disrupted by variations in timing frequency. Nevertheless, signal drift may be introduced into the transmitted timing signals by component failure within phase-locked loops in the distributed components. In a DCC system with parallel planes, such signal drift can be particularly problematic as it is necessary to align the clock signals in each plane prior to performing any diagnostic analysis of equipment performance.
Although it is possible to determine signal drift of a high accuracy timing signal by comparing it to a local timing signal with high accuracy, such high accuracy timing signals require expensive circuitry to implement. If a low accuracy timing signal is used, it would not be possible to determine whether any differences in the measured timing signal were due to signal drift of the high accuracy signal or the inherent inaccuracy of the low accuracy signal.
A similar problem may be encountered with any other system that utilizes a large number of discrete components that receive asynchronous data or control commands from a centralized location. For example, air traffic control systems, cellular telecommunications systems, and distributed controls systems may experience service delays or failure because of data transmission delays.